A Vertical Cavity Surface Emitting Laser (hereinafter may be referred to as “VCSEL”) is a semiconductor laser capable of emitting light in the direction orthogonal to its substrate. When compared with edge emitting semiconductor lasers capable of emitting light in the direction parallel to its substrate, the VCSEL may have some advantages such as lower cost, lower energy consumption, smaller size, and being appropriately applicable to two-dimensionally integrated devices. Recently, because of those advantages, the VCSEL has attracted increased attention.
The surface emitting laser has a current confined structure to enhance current influx efficiency. To form the current confined structure, a selective oxidation process is usually performed on an AlAs (Al: aluminum, As: arsenic) layer. In the following, the current confined structure may also be referred to as an “oxide-confined structure” for convenience (see, for example, Patent Document 1). The oxide-confined structure may be formed by forming a mesa structure having predetermined sizes and having a side surface where a selectively-oxidized layer is exposed. Then, the formed mesa structure is processed under a water-vapor atmosphere so that aluminum (Al) in the selectively-oxidized layer is selectively oxidized from the side surface of the mesa structure. By doing this, an unoxidized region remains in the center portion of the mesa structure. The unoxidized region (hereinafter referred to as a “confined region” for explanatory purposes) becomes a passing region (or a “current passage region”) through which a driving current of the surface emitting laser passes. As described above, the current may be easily contained. The refractive index of the aluminum-oxidized layer (AlxOy) (hereinafter simplified as an “oxidized layer”) in the oxide-confined structure is approximately 1.6, which is lower than that of semiconductor layers. Because of this feature, a refractive index difference is generated in the lateral direction in a resonator structure of the surface emitting laser, and the light is confined in the center of the mesa structure, thereby improving the emission efficiency of the surface emitting laser. As a result, it becomes possible to obtain excellent characteristics such as lower threshold current and higher efficiency.
The surface emitting laser may be generally applied to a light source of an optical writing system in a printer (oscillation wavelength: 780 nm band), a light source of an optical writing system in an optical disk device (oscillation wavelength: 780 nm band and 850 nm band), and a light source of an optical transmission system such as LAN (Local Area Network) using optical fibers (oscillation wavelength: 1.3 μm band and 1.5 μm band). Further, the surface emitting laser is also expected to be used as a light source for optical transmission between boards, within a board, and between chips and within a chip in a Large Scale Integrated circuit (LSI).
In those application fields, it is generally required that a cross-sectional shape of the light emitted from the surface emitting laser (hereinafter referred to as “emitting light”) be circular. To achieve the circular cross-sectional shape, it is required to control higher-order transverse-mode oscillation.
To that end, for example, Patent Document 2 discloses a method of controlling the transverse-mode oscillation by forming an optically transparent film on an emitting surface and differentiating the reflection rates between the center part and its peripheral part of the emitting region.